One of my latest marinades uses pineapple. I have been using it on pork and its really tasty. Tonight I tried it out one some boneless, skinless , chicken thighs and it turned them to complete mush in about an hour! Now I know that pineapple will break down the proteins in meat (or something like that) and has the potential to do this, but I have never seen it happen so fast.

Not sure if you can tell with these pictures but the chicken pretty much fell apart when I flipped em with the tongs. They had a nice flavor but the texture was awful, really mushy.

I always use pineapples in my marinade (on chicken beef, and pork), but this never happened to me before. Were there any other ingredients in your marinade? How much pineapples did you use?
Besides, the enzymes in the pineapples don’t start working until the solution is heated up. The time you soak the meat in a pineapple cold solution has no effect on the meat integrity. Usually the acid in the pineapples will start to cook the chicken, not the enzymes (cooking is not the same as breaking down meat fiber).

My folks brought back some fresh pineapple from Hawaii one year and they were so sweet and so good. But they were also very acidic. If you ate too much they would burn the inside of your mouth. They ripened on the plant and were picked the day they came back, doesn't get any fresher than that. I think ripening on the plant added to the acid levels. The stuff we get here is picked green and shipped to stores. They don't have as much flavor, sweetness or acid. Maybe your pineapple was on the plant a litt longer than it was suppose to be, commercially speaking.

I have used pineapple many times also but never got these results. I used about 1/2 of a fresh pineapple with the juice. The only other acidic item in the marinade was rice wine vinegar. It also had:
A few garlic cloves
Red onions
Brown sugar
A grip of fresh cilantro
Serrano peppers
Kosher salt